Through its robustness, performance, extensive debugging features, clustering functionality, expert community, and through being open source, Gambit Scheme http://www.iro.umontreal.ca/~gambit/ is certainly a very strong candidate. An extensive module system is underway, easing incremental development processes for large projects.

In terms of pre-built libraries, it may be that PLT and Chicken score the highest. Though, like when planning any technology project, you need to ask yourself what libraries are relevant to you, and what purpose they should fill, in detail.

- how they execute code (direct compilation to binary code such as X86 binary code or JIT, compilation to another language such as C, C++ or Javascript, incremental compilation, interpretation) - how they do threading (green threading, operating system threads such as pthreads, none, mixed) - if they fully support continuations - how R5RS and/or R6RS compliant they are - how performant they are - how robust they are - what debuggability features they have - how deep operating system integration they have, and for what operating systems

Scheme has the world's highest number of high-quality software development environments for one and the same programming language.